1. Field of the Invention
The present invention is generally related to a wafer bump fabrication process, and more particularly to a high-lead-content wafer bump fabrication process.
2. Description of Related Art
Semiconductor packaging technology is advancing in the direction of miniaturization and high integration. In terms of high integration techniques such as flip chip, it reduces signal path length for improving signal transmission speed therefore flip chip has become the main method in high integration packaging.
Conventional flip chip techniques usually involve in the processes after redistribution is completed, that is the formation of bumps on the input/output (IO) bonding pads of the wafer. After the singulation of the wafer into chips, the chips are flipped and directly attached to the substrate via the bumps. In order to improve the connection between the bumps and the substrate, a solder paste made containing tin and lead is first coated on the surface of the substrate. Following, after the connection points of the bumps and the substrate are connected, a reflow process is performed to soften the solder paste to complete the connection.
For preventing the bumps and the solder paste from softening together during the flipped chip and substrate connection, a method using high-lead-content bumps is proposed. By using bumps having higher lead content than the solder paste coated on the substrate, the bumps will not soften and deform under the temperature used in the reflow process. However if the lead content of the bumps is increased, correspondingly the temperature used in the reflow process needs to be increased. Such high reflow process temperature (during the formation of the balls) causes a lot of problems which are demonstrated in the followings.
Please refer to FIG. 2-6 and simultaneously refer to FIG. 1 as well, FIG. 2-6 show the schematic sectional view of the bump fabrication process and FIG. 1 show the flow chart of the bump fabrication process.
As illustrated in FIG. 2, a plurality of bonding pads 102 and a passivation layer 104 are located on a wafer 100, where the passivation layer 104 protects the wafer 100 and exposes the bonding pads 102. After an under ball metallurgyunder bump metallurgy (UBM) layer 106 is formed on the bonding pads 102, a dry film (step 122 in FIG. 1) is adhered to the surface of the wafer 100 and then cure to become 108a. Afterwards a curing process (step 124 in FIG. 1) is performed to cure the dry film.
As illustrated in FIG. 3, an exposure development process (step 126 in FIG. 1) is performed to the dry film 108a to form patterned dry film 108b which has a plurality of openings 108c for exposing the UBM layer 106. Afterwards a plasma etching process (step 132 in FIG. 1) is performed to remove the native oxide which is formed on the surface of the UBM layer 106 during the exposure of the UBM layer 106 and to also remove any remaining of the exposure development in the openings 108c. Following, a screen printing process (step 134 in FIG. 1) is performed to fill openings 108c with the solder paste 110 to achieve the structure in FIG. 4, wherein the solder paste mainly comprises solder alloy and flux.
As illustrated in FIG. 5, a reflow process (step 136 in FIG. 1) is performed to increase the temperature to soften the solder paste 110 and to form bumps 110a using the surface tension of the solder paste 110. Afterwards the dry film 108b (step 138 in FIG. 1) is removed as illustrated in FIG. 6, by using the flux in the solder paste to take away the oxide and other contaminants in the ball-shaped bumps to the surface of the ball-shaped bumps, so the bumps become 110b and the reflex becomes 112a. Finally, the flux 112a is removed (step 144 in FIG. 1) and the ball-shaped bumps 110b (not shown) are left.
It is to be noted that when adhering the dry film, if air bubbles exist between the wafer and the dry film the air bubbles will be trapped inside the dry film after the curing process. Afterwards in the exposure development process, the air bubbles still cannot be released. As a result in the subsequent fabrication process until the reflow process which is after filling in the solder paste, problems will occur which are described in details in FIGS. 7-8.
Please refer to FIG. 7, it shows a schematic sectional view of the conventional bump fabrication process during the reflow process with air bubbles exist in the dry film. Wherein reference number 170 represents a wafer, reference number 172 represents a bonding pad, reference number 174 represent a passivation layer, and reference number 176 represents an under ball metallurgyunder bump metallurgy (UBM) layer. If air bubble 173 exists during the adhesion of the dry film, the air bubble 173 will be trapped during the curing process all the way until the solder paste is filled inside the opening 178a and therefore the air bubble 173 is permanently trapped with no escape.
Please refer to FIG. 8, it shows the adverse effects generated by the air bubbles during the reflow process for the structure in FIG. 7. During the reflow process, increasing temperature causes the air bubble 173 to expand in volume to become 173a. If the location of the air bubble 173 is near the opening 178a, the expanded air bubble 173a will push the solder paste 180 to deform and become 180b. As this moment, the solder paste 180b will peel off from the surface of the UBM layer 176 and therefore cannot form ball-shaped bumps 180a, this phenomenon is known as bump missing.
Besides the above described problems, after the dry film is cured the curing of the dry film is adverse so in subsequent screen printing the dry film is easily subjected to deformation which introduces the problem of bump bridge, details are described in FIGS. 9-10.
Please consecutively refer to FIGS. 9 and 10, they show the adverse effect of bump birdge during the conventional bump fabrication process. Wherein reference number 150 represents a wafer, reference number 152 represents a solder pad, reference number 154 represents a solder passivation layer, and reference number 156 represents an under ball metallurgyunder bump metallurgy (UBM) layer. After the conventional curing process, the curing effect of the dry film 160 is not enough so the blade 161 will exert pressure on the dry film 160 causing it to deform and create bump bridge, as illustrated in FIG. 10.
Furthermore the adverse result of the curing can cause other problems. Solder paste is usually made of a combination of flux and solder material, therefore if the curing of the dry film is adverse, the dry film will absorb more flux. In the reflow process, the flux that is absorbed into the dry film will react and form some unwanted reacted material making the dry film hard to be removed.
Summarizing the above, the conventional bump fabrication process has the following disadvantages:
1. Due to the existence of air bubbles between the dry film and the wafer, ball-shaped bumps cannot be formed in the reflow process.
2. Due to the adverse result of the curing of the dry film, the dry film is easily deformed causing bump bridge.
3. The dry film absorbs the flux making the dry film hard to be removed.